Calculate a river system with the field calculator in QGIS

Decoding River Systems with QGIS: It’s Easier Than You Think!

QGIS. It’s not just another piece of software; it’s your digital Swiss Army knife for all things spatial. This open-source Geographic Information System packs a serious punch, offering a ton of tools for digging deep into spatial data. And trust me, one of the coolest? The Field Calculator. It lets you crunch numbers on your data and whip up new fields based on what you already have. Think of it as a spreadsheet on steroids, specifically designed for maps. We’re going to look at how this feature can be used to unlock some secrets of river systems, helping you understand their quirks and characteristics.

First Things First: Get Your Data Ready

Before we get our hands dirty with calculations, let’s make sure our data is prepped and ready to go. Usually, this means having a vector layer that shows your river network. This often comes from a Digital Elevation Model, or DEM. Ideally, your river network should be a polyline layer, where each line is a piece of the river. Think of it as connecting the dots to trace the river’s path.

Let’s Get Calculating!

Here are some essential calculations you can perform using the Field Calculator to analyze your river system:

1. River Length: How Long Is That River, Really?

Calculating the length of those wiggly river segments is a fundamental step. It’s like measuring the lifeline of the landscape. The Field Calculator makes it surprisingly easy, thanks to the $length function.

• Open the attribute table of your river network layer. This is where all the data about your rivers lives.
• Click the “Toggle editing” button. Time to get to work!
• Open the Field Calculator. It’s calculation time.
• Create a new field (e.g., “Length_m”). Give it a name that makes sense and set the data type to “Decimal number (real).”
• Type $length into the expression box. This tells QGIS to calculate the length.
• Hit “OK,” and watch the magic happen as the length of each river segment is calculated.
• Save your edits and exit editing mode. You’re done… for now.

A quick note: If your project’s coordinate system is in degrees, your length will be in degrees too – not very helpful. To get meters, you’ll need to reproject your river network to a projected CRS that uses meters. It’s like changing the units on your measuring tape.

2. Stream Order: Who’s the Boss?

Stream order is like a river’s family tree, showing you how streams connect and grow. The most common method is the Strahler stream order. Headwater streams get an order of 1. When two streams of the same order join, the downstream segment goes up one order. If they’re different orders, the downstream keeps the higher order.

Okay, the Field Calculator can’t directly calculate stream order in one shot. But don’t worry! We can use it with other QGIS tools or plugins. One way is with the “Stream Order” tool in the SAGA GIS toolbox within QGIS. This tool needs a flow direction grid from a DEM.

• Make sure you have a DEM for your area.
• Use the “Fill Sinks” tool (SAGA > Terrain Analysis – Hydrology) to fill any dips in the DEM. It’s like smoothing out the wrinkles.
• Use the “Flow Direction” tool (SAGA > Terrain Analysis – Hydrology) to figure out which way the water flows from the filled DEM.
• Use the “Stream Order” tool (SAGA > Terrain Analysis – Hydrology) with the flow direction grid. Tell it where to save the stream order raster.
• Once you have the stream order raster, turn it into a vector layer using the “Raster to Vector” tool (Raster > Conversion).
• Finally, if needed, use the Field Calculator to copy the stream order values from the vector layer to your original river network layer based on location.

Alternatively, the Hy2roresO plugin can also calculate stream order.

3. Flow Accumulation: Where’s the Water Going?

Flow accumulation tells you how many upstream cells drain into a cell. It’s super useful for finding areas with lots of water and potential flood zones.

Like stream order, flow accumulation needs specialized tools, not just the Field Calculator. The SAGA GIS toolbox has the “Flow Accumulation” tool (SAGA > Terrain Analysis – Hydrology) for this.

• Make sure you have a DEM and have filled sinks and calculated flow direction.
• Use the “Flow Accumulation” tool, using the flow direction grid as input.
• Name your output file for the flow accumulation raster.
• The result is a raster showing how many upstream cells drain into each cell.

4. Drainage Density: How Efficient Is the River?

Drainage density is the total length of streams per area. It shows how well water moves through a catchment.

• Calculate the catchment area using the Field Calculator and the $area function on the catchment polygon layer.
• Calculate the total length of the stream network within the catchment as we did earlier.
• Use the Field Calculator to make a new field (like “DrainageDensity”) and calculate it with the formula: StreamLength / CatchArea.

5. River Slope: How Steep Is That River?

River slope, or gradient, is the change in elevation over a distance. You can estimate it using a DEM and the river network.

• Extract elevation values from the DEM to the river network as a new attribute. You can do this using the “Sample raster values” tool (Processing Toolbox > QGIS geoalgorithms > Raster analysis).
• Split the river lines into smaller segments using “v.split.length” tool in the Processing Toolbox.
• Calculate the slope for each segment using the Field Calculator with the expression (z(end_point($geometry)) – z(start_point($geometry))) / $length. This calculates the slope as the change in elevation divided by the length of the segment.

Level Up: Advanced Calculations

The Field Calculator isn’t just for basic stuff. You can use it for more complex analyses, such as:

• Calculating distances to rivers: Using the “Distance to nearest hub (lines)” tool in the QNEAT3 plugin to calculate the distance from any point to the river network.
• Cumulative sums of river flow: Calculating the cumulative flow downstream, considering potential confluences.
• Land cover percentages per subcatchment: Calculating the percentage of different land cover types within each subcatchment.

Wrapping Up

The Field Calculator in QGIS is a fantastic tool for understanding river systems. Paired with other QGIS tools and data like DEMs, you can find out a ton about river length, stream order, flow, drainage, slope, and more. These calculations are useful in many fields, from hydrology to ecology to environmental management. So dive in, experiment, and see what you can discover!